Check how many cables or conductors fit in a conduit and the percentage fill against the 40% maximum, for UK and US conduit sizes.
The calculator compares the total cross-sectional area of your cables against the permitted fill. The standard fill limits are 53% for a single cable, 31% for two cables and 40% for three or more cables. Staying within these limits prevents overheating and makes the cables easier to pull.
Both BS 7671 (UK) and the NEC (US) use the same percentages: 53% for one cable, 31% for two cables and 40% for three or more cables. These limits leave room for heat dissipation and easy pulling.
A conduit packed too tightly traps heat, makes cables hard to pull without damaging the insulation, and leaves no room for future additions. The 40% rule is a long-standing safety margin.
The overall diameter is printed on the cable or in the manufacturer data sheet. It is the diameter over the outer sheath, not the conductor itself.
Yes. Switch the standard to choose metric conduit sizes (16–50 mm) or US trade sizes (1/2" to 2"). The internal area is built in for each size.
This calculator assumes identical cables. For mixed cables, add up each cable’s individual area separately and keep the total below 40% of the conduit area.
